1. Field of the Invention
The present invention relates to the mechanical arts and methods that embody underwater work methods. More particularly, it relates to devices and methods for improving the productivity of a remotely operated vehicle (ROV) engaged in underwater maintenance and construction work.
2. Description of Related Art
Conventional underwater work techniques often include the use of remotely operated vehicles (ROV's). A surface support vessel and its associated personnel support and operate the ROV. The ROV may be deployed directly from the support vessel or from the surface via a tether management system (cage). When deployed directly from the surface, the ROV is connected to its control and powering components on the support vessel with an umbilical cable. When deployed from the surface in a cage, the cage and ROV are lowered to a location near the worksite on a similar umbilical cable. Thereafter, the ROV may be maneuvered from the cage to the worksite while coupled to a tether extending between the ROV and the cage.
Regardless of the method employed to deploy the ROV to the worksite, ROV's are designed so that they are essentially neutrally buoyant (they neither float nor sink). Therefore, addition or removal of payloads (weight) to/from the ROV requires that the ROV have either excess thrust capacity or the ability to add or remove buoyancy or ballast to compensate for the addition or removal of weight.
ROV operations include use at an underwater worksite to manipulate various payloads. Supporting payloads with a specific gravity (SG) greater than unity tends to make the ROV sink. Supporting payloads with a SG less than unity tends to make the ROV float. Because of this, the ROV must be able to compensate for or manage its buoyancy when on-loading or off-loading a payload.
A typical ROV utilizes fixed buoyant volumes such as syntactic foam or fixed air voids in combination with its vertical thruster's capacity to manage its buoyancy relative to the ROV equipment's weight or negative buoyancy. When large packages are added to the ROV, the package's buoyancy is typically compensated for via fixed buoyant volumes or ballast tanks added to the package at the surface, thereby enabling the ROV to manage the package's buoyancy. The ballast tank may be filled with gas or liquid or a combination of both. Replacing liquid with gas in the ballast tank makes the ROV rise while replacing gas with liquid tends to make the ROV sink. Typically, the gas is air and the liquid is water.
When on-loading a dense payload (SG>1) the ROV's buoyancy may be adjusted by replacing liquid with gas (deballasting) in the ballast tank. To compensate for off-loading the dense payload, the ROV's buoyancy may be adjusted by replacing gas with liquid (ballasting) in the ballast tank. Conversely, to compensate for on-loading a scant payload (SG<1), the ROV's buoyancy may be adjusted by replacing gas with liquid. The ROV's buoyancy may be adjusted to compensate for offloading the scant payload by replacing liquid with gas.
The ROV consumes compressed gas from an integral (onboard) gas storage system each time it performs the deballasting operation. When the integral gas storage supply is depleted, it must be replenished. The ROV must return to the surface for gas replenishment. A remote operator maneuvers the vehicle back to the surface, either directly or via the cage, where surface vessel resources replenish its integral gas storage system. Redeployment of the ROV is in either case accomplished by reversing the recovery operations.
ROV productivity is significantly reduced when it is employed to repetitively move payloads from one location to another. Repeated on-loading and off-loading of payloads requires repeated gas recharge operations which deplete the ROV's integral gas storage supply. The ROV is therefore required to make frequent trips to the surface to replenish this supply. Such trips to the surface consume time and are inefficient, regardless of how the ROV is deployed.
Accordingly, there has existed a need for improved ROV buoyancy control systems. There is a still further need for improved ROY work methods. The present invention satisfies these and other needs, and provides further related advantages.